It is known in the art of eye care to correct abnormal vision by use of lenses and to alter focus of an image on the retina of the eye. Such lenses can be worn on the surface of the eye as contact lenses or implanted within the eye as intraocular lenses. The lenses can also be used on a camera to enhance depth of focus.
Contact and intraocular lenses attempt to shift the plane of focus to coincide with the retinal plane. In the case of near sightedness, the plane of focus is forward of the retinal plane and the lenses shift the focused image rearwardly toward the retinal plane. In the case of far sightedness, the plane of focus is rearward of the retinal plane and the lenses shift the focused image forwardly toward the retinal plane.
In cases where a patient suffers from both near sightedness when viewing distant objects and far sightedness when viewing near objects, e.g., when reading, lenses which are contoured to remedy one of the deficiencies tend to exacerbate the other. Typically such a patient will be required to use a corrective lens having a negative diopter power in order to obtain desirable distance vision, e.g., 20/20 at infinity. For reading, the patient will be required to add a positive diopter power. However, the addition of the positive diopter power will degrade the distance vision, thereby requiring that the add power lens be removable, e.g., in the form of eye glasses.